OBJECTIVE--To compare the effectiveness over three years of chiropractic and hospital outpatient management for low back pain. DESIGN--Randomised allocation of patients to chiropractic or hospital outpatient management. SETTING--Chiropractic clinics and hospital outpatient departments within reasonable travelling distance of each other in 11 centres. SUBJECTS--741 men and women aged 18-64 years with low back pain in whom manipulation was not contraindicated. OUTCOME MEASURES--Change in total Oswestry questionnaire score and in score for pain and patient satisfaction with allocated treatment. RESULTS--According to total Oswestry scores improvement in all patients at three years was about 29% more in those treated by chiropractors than in those treated by the hospitals. The beneficial effect of chiropractic on pain was particularly clear. Those treated by chiropractors had more further treatments for back pain after the completion of trial treatment. Among both those initially referred from chiropractors and from hospitals more rated chiropractic helpful at three years than hospital management. CONCLUSIONS--At three years the results confirm the findings of an earlier report that when chiropractic or hospital therapists treat patients with low back pain as they would in day to day practice those treated by chiropractic derive more benefit and long term satisfaction than those treated by hospitals.